Loading lumber to stress levels at some factor above its design value is a useful method for proving its strength. Boards with naturally occurring strength-reducing characteristics such as knots and decay, or with distorted grain or man-made characteristics such as finger joints or machining inaccuracies, if severe enough, can be failed by loading them prior to use. By this method, substandard material can be removed from use, increasing the minimum strength in the population of surviving boards.
Because of lower costs, smaller force, and lack of a clamping requirement, most testing and research of lumber strength properties has been in bending. However, in many cases tension is the preferred strength property to prove or measure. Tension loading offers a more thorough test of all elements of the material cross-section than does bending. There is less risk of testing-induced incipient failures with tension than with bending. This includes the risk of causing compression wrinkles in lumber and the risk of weakening the outer fingers in the joints of horizontally finger-jointed material. In addition, the duration of maximum load is more easily controlled in the production line with tension than with bending test apparatus. In many cases of interest for structural lumber, such as for the lower chords of trusses, tension testing more nearly simulates the actual condition of use.
Any improvement in tension testing apparatus which reduces the cost and eases the clamp complexity will make tension testing more practical and thus bring it into more generally use. Because the clamping mechanism has been the most costly part of tension proof testing apparatus for lumber, it follows that clamp innovation is important.
The present disclosure presents an apparatus for gripping lumber with clamps such that the clamping force is not so high as to crush the material nor so low as to allow slippage when the tensile load is applied. This apparatus can be used for tensile testing of lumber to provide the strength of individual pieces of lumber or wood timers at high tensile loads. Tested material can be used with greater confidence in its reliability for critical structural applications. The tension testing apparatus can also be used for researching the tensile strength of materials and for production quality control testing.
To apply a tensile force to material such as lumber, one must first grip it sufficiently well at two places with clamps. When tensile force is applied, the clamps must not slip along the lumber surfaces. The tensile force that can be applied before slippage occurs is a function of the clamping force and the coefficient of friction at the wood/clamp interface. The condition that must be satisfied is: EQU T&lt;2C.mu..sub.w
where T is the tensile force applied, C is the clamping force applied in the direction perpendicular to the wood/clamp interface, and .mu..sub.w is the coefficient of friction at the wood/clamp interface. The factor "2" enters this relation because there are two wood/clamp interfaces. Also important, when dealing with crushable material such as wood, is its crushing strength, or the amount of clamp pressure the wood material can sustain before it is crushed by the clamps.
For a given tensile force T to be allowed, one must make the friction .mu..sub.w and/or the clamping force C large enough so that the above relation is satisfied. But the clamping force C must not be so large as to cause crushing.
One method used in previous designs for increasing the friction has been to use serrated metal grip plates; then when clamp pressure is applied, the teeth bite into the material surface. U.S. Pat. Nos. 3,556,480, 3,685,801, and 3,763,654 utilize this method for clamping material. Although serrated grip plates greatly increase the effective friction, they also cause damage to the surfaces of the test specimen.
The best clamp gripping surface presently known which does not cause damage to wood but yet has a reasonably high friction against wood is provided by a soft (70-90 durometer) polyurethane coating applied to a steel backing, the polyurethane serving as the gripping interface. The polyurethane-to-wood interface has enough friction to prevent slippage of the grips provided the force C is large enough. But, the range of acceptable forces is limited because crushing of boards will occur if the force is too large.
The acceptable clamp force range can be increased by increasing the clamp contact area. Practically, this is accomplished by increasing the length of the clamps. The tradeoff the length of the material clamped versus the length tested. Also, the larger the clamp area, the more difficult and expensive it is to apply a uniform clamping pressure over this area.
Test specimens of lumber that will sustain the higher tensile loads also will usually sustain higher clamp forces without crushing. Previous clamp designs have made use of this fact by using the same hydraulic pressure to actuate the clamp force that actuates the tensile force in a linear relationship. Then, as the tensile force increases, so also does the clamp force. The clamp force to tensile force ratio is controlled by choice of the actuating hydraulic cylinder areas. This method has proved to be technically acceptable, but it is very expensive because of the hydraulic apparatus required. It has also been observed experimentally with this type of equipment that for the higher tensile forces, the clamp force to tensile force ratio required to prevent slippage increases. To achieve the required ratio at high tensile forces, one must increase the ratio over the entire force range because the hydraulic cylinder areas cannot be adjusted in use.
Another method for achieving a constant clamp force to tensile force ratio uses a mechanical wedge arrangement so that with application of tesnile force, grip plates moving along the inclined planes of wedge members also move perpendicularly to the tensile force direction thereby causing a clamping force. The amount of clamping force is dependent on the angle of the inclined planes, the coefficient of friction between the grip plate/wedge member interfaces, and the tensile force applied. Use of wedges in one form or another can be found in the following U.S. Pat. Nos.: 839,784; 2,831,654; 2,989,337; 3,168,205; 3,170,322; 3,556,480; 3,685,801; 3,763,654; 3,774,352; 3,815,117; 4,050,675; 4,053,255; 4,208,045; 4,365,792; 4,410,169; and 4,506,871. The wedge clamp method for tension testing has been applied to off-line quality control testing of lumber and other wood products. Advantages of the mechanical wedge arrangement are that fewer parts are required, the inclined planes can be segmented and spread out over the gripping surfaces so that the clamp force is distributed over the grip area, the clamps can tolerate partial coverage of their lengths by the material being tested without damaging the clamp parts, and expensive hydraulic equipment is eliminated.
Disadvantages of prior art mechanical wedge clamps have been problems of friction. The lack of consistency of the coefficient of friction between the pressure plate and the indicated plane has caused inconsistency in the clamp force relative to the tensile force. Consequently, either the clamps sometimes slip on the wood, or the inclined plane angle is adjusted for a higher clamp force to tension force ratio than would otherwise be necessary. This in turn leads to wood fiber crushing problems. Another friction problem is that for the materials which have been used in the grip plate/wedge member interface, static friction is greater than dynamic friction. This can lead to clamp lockup at high force levels. When trying to unload the clamps, they can stick, sometimes requiring the use of sledgehammers to open them. When large tensile forces are applied at very low velocity, the resulting large static friction forces can prevent an increment of tensile force from causing a corresponding increment of clamping force. Thus, particularly at high force levels, slippage of wood in the clamps can occur.
The present invention utilizes a multiple split wedge clamping arrangement for tension testing of lumber. This arrangement uses polytetrafluoroethylene (PTFE) bearings between the moving and stationary wedge members to solve the friction problems that have been seen with other wedge clamps used for tension testing. The properties of PTFE bearings as used in the wedge clamps are uniquely matched to the clamping requirements of lumber provided that the bearing wedge angle and clamp area are properly chosen.
The objective of the invention is to provide a low cost, high reliable, low maintenance clamp apparatus for tension testing of lumber. Lumber can be crushed if subjected to too much clamp pressure, but will slip in the clamps if the clamping pressure and/or the coefficient of friction of the gripping surface against the wood is not sufficiently high.
Although PTFE sliding against steel has a relatively large coefficient of friction when compared with roller or ball bearings, its reduced coefficient of friction with increased load, its static values that are no higher than its dynamic friction values, and the designer's ability to choose its sliding contact bearing load area and wedge angle make PTFE an ideal solution as the bearing material between relatively moving wedge members in this application. Further, PTFE is a low cost solution and its natural viscoelastic deformation characteristics under load allow it to conform to manufacturing tolerances of less precision than are required for rolling bearings. The method described for computing bearing area causes large bearing pressures at rated tension load for the clamps. This is the minimum coefficient of friction situation and is shown to correspond to a maximum clamp-to-tension force ratio. Experimentally, higher clamp-to-tension force ratios are needed at high loads than at low loads to prevent slippage of lumber. The decreasing coefficient of friction with increasing load makes PTFE an ideal choice of bearing material to match the clamping force versus load requirements of lumber. Such matching requires proper choice of both bearing area and wedge angle according to the described procedure.
Another advantage of the wedge clamp apparatus with PTFE bearings is the absence of clamp lockup. Clamp lockup occurs as a result of "stiction" which occurs when the static coefficient of friction for a bearing material is larger than its dynamic coefficient of friction. Practically all bearing materials except PTFE exhibit some stiction.
Use of wedge clamps is more efficient in the production-line tension testing of lumber than the use of hydraulic clamps. For fast operation, hydraulic clamps need a separate low pressure, high flow hydraulic circuit for fast clamp action until the clamps contact the wood; then it is necessary to switch to high pressure and low flow. Accumulators, valving and controls to accomplish this are complex and expensive when compared to the simplicity of using an air cylinder or other simple low cost actuator for the "close" and "open" functions of the wedge clamping apparatus.